Memory elements are key components of all the electronic central processing units (CPU) as for example shift registers within the arithmetic logic units to perform operations as multiplication and division. CPUs are connected with random access memory (RAM) slots to store and read data to be processed. Telecom applications require memory elements as well. Buffers within the network nodes store information when a contention occurs at an output port. Shift registers are employed for serial to parallel conversion and cyclic operations. Error detection and correction techniques such as parity check, or cyclic redundancy check can be implemented with shift registers. Linear feedback shift registers (LFSR) enable encryption/decryption within secure communication systems. They are also employed for scrambling data streams to be transmitted, and for pseudo-random bit sequence (PRBS) generators. The possibility to implement these important functionalities in the optical domain could improve the performance of future optical networks. In fact they could allow storing of information without converting the data into the electronic domain, thus avoiding the speed limitations typical of electronics. Another issue of the present network nodes is the huge amount of power consumption, complexity and footprint. The chance to operate signal processing into the optical domain, included storing information, could help to reduce the power consumption, complexity and footprint if integrated solutions can be adopted. Optical elementary logic gates, complex digital logic functions and their applications within an all-optical packet switching node are already demonstrated. Despite optical computing still being some way from commercial use, the demonstration of optical logic circuits and optical memory elements with discrete components represents a first step. For practical implementations integrated solutions are desirable to reduce productions costs. It is known from U.S. Pat. No. 6,952,172 to provide an optical PRBS generator using optical delays, an optical feedback path and an optical XNOR gate. It is also known to provide an all optical linear feedback shift register using a non recirculating shift register, as shown in T. Houbavlis, K. E. Zoiros, M. Kalyvas, G. Theophilopoulos, C. Bintjas, K. Yiannopoulos, N. Pleros, K. Viachos, H. Avramopoulos, L,. Schares, L. Occhi, G. Guekos, J. R. Taylor, S. Hansmann, W. Miller, “All-Optical Signal Processing and Applications Within the Esprit Project DO_ALL”, IEEE J. Lightwave Technol., vol. 23, n.2, pp. 781-801, February 2005. This uses an optical regenerative memory so the optical pulses are sent once round a loop, and then are regenerated at the UNI device.